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<div class="titlepage"><div><div><h3 class="title">
<a name="boost_asio.reference.experimental__promise"></a><a class="link" href="experimental__promise.html" title="experimental::promise">experimental::promise</a>
</h3></div></div></div>
<p>
        <a class="indexterm" name="boost_asio.indexterm.experimental__promise"></a>
      </p>
<p>
        A disposable handle for an eager operation.
      </p>
<pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span>
    <span class="keyword">typename</span> <span class="identifier">Signature</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">(),</span>
    <span class="keyword">typename</span> <a class="link" href="Executor1.html" title="Executor requirements">Executor</a> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">any_io_executor</span><span class="special">,</span>
    <span class="keyword">typename</span> <span class="identifier">Allocator</span> <span class="special">=</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">allocator</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;&gt;</span>
<span class="keyword">struct</span> <span class="identifier">promise</span>
</pre>
<h5>
<a name="boost_asio.reference.experimental__promise.h0"></a>
        <span class="phrase"><a name="boost_asio.reference.experimental__promise.types"></a></span><a class="link" href="experimental__promise.html#boost_asio.reference.experimental__promise.types">Types</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody><tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/value_type.html" title="experimental::promise::value_type"><span class="bold"><strong>value_type</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  The value that's returned by the promise.
                </p>
              </td>
</tr></tbody>
</table></div>
<h5>
<a name="boost_asio.reference.experimental__promise.h1"></a>
        <span class="phrase"><a name="boost_asio.reference.experimental__promise.member_functions"></a></span><a class="link" href="experimental__promise.html#boost_asio.reference.experimental__promise.member_functions">Member
        Functions</a>
      </h5>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
                <p>
                  Name
                </p>
              </th>
<th>
                <p>
                  Description
                </p>
              </th>
</tr></thead>
<tbody>
<tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/cancel.html" title="experimental::promise::cancel"><span class="bold"><strong>cancel</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Cancel the promise. Usually done through the destructor.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/completed.html" title="experimental::promise::completed"><span class="bold"><strong>completed</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Check if the promise is completed already.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/operator_lp__rp_.html" title="experimental::promise::operator()"><span class="bold"><strong>operator()</strong></span></a>
                </p>
              </td>
<td>
                <p>
                  Wait for the promise to become ready.
                </p>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/promise.html" title="experimental::promise::promise"><span class="bold"><strong>promise</strong></span></a> <span class="silver">[constructor]</span>
                </p>
              </td>
<td>
              </td>
</tr>
<tr>
<td>
                <p>
                  <a class="link" href="experimental__promise/_promise.html" title="experimental::promise::~promise"><span class="bold"><strong>~promise</strong></span></a> <span class="silver">[destructor]</span>
                </p>
              </td>
<td>
                <p>
                  Destruct the promise and cancel the operation.
                </p>
              </td>
</tr>
</tbody>
</table></div>
<div class="variablelist">
<p class="title"><b></b></p>
<dl class="variablelist">
<dt><span class="term">Signature</span></dt>
<dd><p>
              The signature of the operation.
            </p></dd>
<dt><span class="term">Executor</span></dt>
<dd><p>
              The executor to be used by the promise (taken from the operation).
            </p></dd>
<dt><span class="term">Allocator</span></dt>
<dd><p>
              The allocator used for the promise. Can be set through use_allocator.
            </p></dd>
</dl>
</div>
<p>
        A promise can be used to initiate an asynchronous option that can be completed
        later. If the promise gets destroyed before completion, the operation gets
        a cancel signal and the result is ignored.
      </p>
<p>
        A promise fulfills the requirements of async_operation.
      </p>
<h5>
<a name="boost_asio.reference.experimental__promise.h2"></a>
        <span class="phrase"><a name="boost_asio.reference.experimental__promise.examples"></a></span><a class="link" href="experimental__promise.html#boost_asio.reference.experimental__promise.examples">Examples</a>
      </h5>
<p>
        Reading and writing from one coroutine.
      </p>
<pre class="programlisting"><span class="identifier">awaitable</span><span class="special">&lt;</span><span class="keyword">void</span><span class="special">&gt;</span> <span class="identifier">read_write_some</span><span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">ip</span><span class="special">::</span><span class="identifier">tcp</span><span class="special">::</span><span class="identifier">socket</span> <span class="special">&amp;</span> <span class="identifier">sock</span><span class="special">,</span>
    <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">mutable_buffer</span> <span class="identifier">read_buf</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">const_buffer</span> <span class="identifier">to_write</span><span class="special">)</span>
<span class="special">{</span>
  <span class="keyword">auto</span> <span class="identifier">p</span> <span class="special">=</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_read</span><span class="special">(</span><span class="identifier">read_buf</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">use_awaitable</span><span class="special">);</span>
  <span class="identifier">co_await</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">async_write_some</span><span class="special">(</span><span class="identifier">to_write</span><span class="special">,</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">asio</span><span class="special">::</span><span class="identifier">deferred</span><span class="special">);</span>
  <span class="identifier">co_await</span> <span class="identifier">p</span><span class="special">;</span>
<span class="special">}</span>
</pre>
<h5>
<a name="boost_asio.reference.experimental__promise.h3"></a>
        <span class="phrase"><a name="boost_asio.reference.experimental__promise.requirements"></a></span><a class="link" href="experimental__promise.html#boost_asio.reference.experimental__promise.requirements">Requirements</a>
      </h5>
<p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">boost/asio/experimental/promise.hpp</code>
      </p>
<p>
        <span class="emphasis"><em>Convenience header: </em></span>None
      </p>
</div>
<div class="copyright-footer">Copyright © 2003-2023 Christopher M. Kohlhoff<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div>
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